In multistage pulp bleaching and purification processes wherein wood pulp is subjected to a plurality of bleaching and caustic extraction stages, typically using mixtures of chlorine dioxide and chlorine in the first bleaching stage, chlorine dioxide in subsequent bleaching stages and sodium hydroxide in the caustic extraction stages, and the pulp is washed intermediate each such bleaching and caustic extraction stage, the volume of effluents from the multistage bleaching process and the overall fresh water requirement are decreased by effecting countercurrent washing operations within the multistage process and by using effluents from the bleach plant to wash the unbleached pulp during to its passage from the digester to the bleach plant, preferably utilizing the procedure described in U.S. Pat. No. 4,039,372.
One unforeseen problem which arises when the latter operations are used is that, to achieve the same final pulp brightness, chemical consumption in the first bleaching stage is increased when compared with bleaching effected in the absence of such use of effluents from the bleach plant. The increase in chemical consumption has been found to result from the presence of dissolved organic material in the aqueous phase of the dilute pulp suspension. The dissolved organic material consumes some bleaching chemicals, thereby, increasing chemical consumption.
The term "dissolved organic material" as used herein refers to bleaching chemical-consuming organic material dissolved in the aqueous phase of the pulp suspension and is measured in terms of total organic carbon (TOC).